Accurate tower light monitoring is a serious matter, and great care can
be taken to install a thorough and automatic monitoring system. Designing
a system to provide a single-lamp failure can quickly fill several status
channels on a facility's transmitter remote control system if one status
input is dedicated to each lamp monitor circuit. Here is a method to spare
the extra status channels and instead use a single telemtry channel to
provide the same level of lamp resolution.

This pseudo-binary
system uses a series of diodes and relay contacts to vary the indicated
voltage. The parts needed:1K, 1W resistor12vdc power supply7
diodes, 1N4005 are used here3 SSAC tower light monitor
modules

The
SSAC tower light monitor modules have floating form C relay contacts.
These are installed so that the relay contacts are closed when the lights
are off. The 1N4005 diodes have higher reverse-voltage and forward-current
ratings than this circuit requires, but because they are so inexpensive,
the higher ratings provide additional insurance against
failure.

The relay contacts in the diagram are connected as
follows:S1 - Top beaconS2 - Center beaconS3 - Side
markers

Because the voltage drop across the diode junction is 0.7
volts, the resulting output voltage of the circuit can easily identify
which lights are on or off. When all the tower lights are off, the circuit
output is zero volts. When only the side lights are on, the circuit output
is 0.7 volts. When only the center beacon is on, the circuit output is 1.4
volts. When only the top beacon is on, the circuit output 2.8
volts.

Because the diodes and switches are in series, the voltages
will add if more than one relay is open. Calibrate the remote control
telemetry input for any desired multiplication factor. A handy reference
in this example would be to set an input voltage of 0.7 volts to provide a
100mv reading. With this, the resulting telemetry readings based on the
lamp condition would be:0 Volts = All Lights OFF100 = side lights
ON, both beacons OUT200 = center beacon ON, top beacon and side lights
OUT300 = side lights and center beacon ON, top beacon OUT400 = top
beacon ON, center beacon and side lights OUT500 = top beacon and side
lights ON, center beacon OUT600 = top beacon and center beacon ON,
side lights OUT700 = All lights ON

These divisions can also be
used to set various alarm levels to indicate a lamp failure. If the
reading drops below 650 but stays above 550, a single side marker is out.
The FCC and FAA do not require notification of a single side marker, so
this would be a warning indicator. However, if the reading drops below
550, a more serious alarm can be set because at least one beacon has
failed.

This diode-string scheme could be continued with more
switches and sets of diodes, but the next step would be require 8 diodes,
and then 16 diodes and so on. This can quickly get out of hand. In an
instance where more than 4 levels of metering are required, reverse-biased
zener diodes would be a better choice.

Moates operates Loud and Clean, a company that designs, installs,
optimizes and maintains radio broadcast antenna systems, transmission
facilities, audio processing systems, digital storage and production
facilities and studio architecture and acoustics. He is based in the
Boston area. Reach him through http://www.loudandclean.com/